GeoGen Script reference
GGE2604

Specified index was not found in the array.

Index into an array is a numeric identifier of position of a key-value pair in the array from first pair (index 0) to the last (index Array.Count() - 1).

var array = {"a": 1, "b": 2 };
array.GetKeyByIndex(1); // OK, returns 2
array.GetKeyByIndex(2); // GGE2604